In the two decades and change since Metallica's landmark Master of Puppets album debuted, heavy metal has charted an ascendant course, from derided, marginalised and supposedly ridiculous youth cult to a commercial behemoth enjoying both mainstream acceptance and critical approval. But while the scene's pioneers continue to plot lucrative victory laps via the globe's enormodomes, subsequent generations have balkanised metal into myriad subgenres (Wikipedia lists 19 "primary" subgenres, leaving aside secondary sub-genres) leaving its loyal audience riven with tribal differences, unable to agree upon a true successor to the throne.

It's against such a backdrop that Mastodon, a grizzly and marvellous quartet from Atlanta, Georgia, have been acclaimed as metal's saviours. A portrait in paradoxes - cerebral enough to have rewritten Melville's Moby Dick in thrash for 2004's Leviathan album, heavy enough to craft crushingly cro-magnon riffs - Mastodon's sound reunites metal's atomised components, running the genre's gamut from glacial heaviness to lightspeed tumult, with an ambition that sets them apart from their peers. Their new album, Crack the Skye, is their most accessible yet - though this doesn't preclude songs with four "movements" and a 15-minute closing track that detours into Zappa-esque skronk - and the group plan to spend the summer touring alongside Metallica who, when interviewed for Film&Music last year, named Mastodon as the preferred heirs to their throne.

"That's quite a compliment to swallow, right there," grins guitarist Bill Kelliher, who first heard Master of Puppets as a 15-year old Rush fan, and was inspired to form a high-school metal covers band, named Crinkled Pig after a type of leather. "Their records sounded like classical music, but played with heavy guitar; it blew my mind, so deep and complex."

"We are literally here because they existed," adds Troy Sanders, who was inspired to pick up his instrument by Metallica's late bassist Cliff Burton. "I decided that being a bass player, feeling the rumble and living that lifestyle would be the coolest thing on Earth. So I taught myself to play by jamming along with Metallica's records, they were like Bass Playing 101. Now, it feels like everything's come full circle."

Singer-guitarist Brent Hinds is less starry-eyed. "They still put their pants on one leg at a time," he says of his patrons, though he adds that he still rocks out to their earlier records, "which I have on vinyl. I had to stop listenin' to them after the Black Album." Hinds also shrugs off suggestions that Mastodon have "saved" metal, or that Crack the Skye is even a metal album. "It's classic rock," he asserts, "And that's the direction I've always wanted us to take. This is the first album where I wrote all the music, and I never wanted us to be this screaming, lumbering, lotsa-drum-solos band, I always wanted us to be this heavy, psychedelic thing."

When Hinds discovered Master of Puppets, aged 13, he was already a proficient banjo player, and that influenced his fret-hammering approach to the electric guitar. He was also, he says, "a total hellion, straight outta hell, with red eyes and everythin'. I grew up in Alabama, in Birmingham. Boring-ham. I was very dysfunctional at school, just a jackass. I'd take LSD and come to class still tripping. I was too creative, never doing my homework, just filling my notepad up with drawings of skulls. I was probably like every other teenager, to be honest with you."

Hinds and Sanders are both veterans of go-nowhere bands, and met when the latter's previous band came to Birmingham to play. "This wild homeless-looking goon was at the show, who turned out to be Brent," says Sanders. "He immediately expressed an interest in coming up to Atlanta, two hours away, to jam with us. A week or so later, he'd moved over and we started making music together."

"That's when I started getting really serious about music, because I had given up everything else and moved to Atlanta," says Hinds. "Music was what I was gonna do. I'm real stubborn, you know?"

Kelliher and drummer Brann Dailor were serving as the rhythm section for the influential math-metal group Today Is the Day when Kelliher became aware of Hinds's prodigious guitar-playing. "The day we quit that band, we moved to Atlanta and met up with these goofuses," he remembers. "I'd already seen Troy and Brent play, and I was, like, wow. Troy had a van and we cooked up some songs, and within three months, we were touring."

The group signed to the underground metal imprint Relapse in 2001, and displayed their ambition - and a gift for brutal metal and prog complexity - from the off. But while metal is infamously fond of flashy instrumental bravado, Mastodon's dizzying flourishes never came at the sacrifice of songcraft.

"Our music is very arpeggiated, has a lot of swells and movement and spiralling chord arrangements." says Hinds. "I studied classical guitar for a year at the Alabama School of Fine Arts, and once I got to a certain level of playing, I got to a certain level of writing music. You don't write at elementary level any more, you're writing kinda complex stuff, because you're a more complex player. But we marry the riffs together, and make sure the riffs are all 'friends', that they're gonna sit together in a song. It takes a while to find the right riffs to be room-mates, though."

Their second album, Leviathan, didn't take Moby Dick as its inspiration to impress rock critics, but because the group identified with Ahab's grueling quest. "Leviathan was about chasing the whale, the sacrifice we made every day," says Kelliher. "We had a big white van, which we all piled into and kissed our families goodbye, just like the sailors going off to chase that whale." Similarly, 2006's Blood Mountain played out as a metaphor for their signing to Warner Bros. "Warner Bros was this huge entity, the mountain, and now we're climbing to the top of it. Where do we go from there?"

With graceful, dark harmonies and sweeping symphonic melodies, Crack the Skye takes the group in a new direction. Again, the album takes events from the group's private lives as inspiration for its lyrical flights of fantasy, primarily the serious head injury Hinds sustained at the 2007 MTV video music awards, leaving him in a coma, threatening both his life and Mastodon's future.

"Some jackass sucker-punched me," growls Hinds. "He already had brain damage," laughs Kelliher, before getting serious again. "We thought, 'Shit, is he gonna be a total vegetable when he gets out of the hospital?' We were just hoping for the best, and kinda bummed out."

"I was asleep for like three days," remembers Hinds, "and was having some crazy out-of-body experiences. I showed signs of waking up, so the doctors took the catheter out of my penis, and when they did that, I woke up and projectile vomited over everybody. And I wish they hadn't done that, because it felt really good to be asleep for that long. Afterwards, I was laid up on the couch for months, with really extreme, euphoric vertigo. The whole entire time I was carving wood, getting tattoos and playing my guitar. I was going to band practice, with vertigo. I don't think I ever really let on to myself, about how serious the injury was. I'm stubborn. My dad says, 'You can't kill stupid.' I wouldn't stay down; they couldn't keep me down."

The rest of Mastodon share Hinds's no-compromise spirit. "We're not underdogs," says Kelliher, "But we did everything from the ground up, we built what we have today in blood, sweat and tears. Somebody didn't just hand us this shit. And I like hearing that kids think we've 'saved' metal, but I can't entirely agree. We're just part of a movement that's changing your average metal band's sound. And it's getting harder for me to accept we're a metal band. I feel we're a progressive rock band. I think we're getting bolder and more ambitious as writers, maturing, letting go of our inhibitions. That's what bands have to do, you shouldn't try to make the same record twice."

For the stubborn Hinds, meanwhile, the Grammys and platinum records and media attention Mastodon have earned are all further justification for the path he's chosen in life.

"I remember going home for Christmas and saying to my parents, 'Lookit, this is my new band, I want you to listen to it,'" he says. "And they'd say, 'That's nice, honey, when are you gonna go to school and get a proper job?' I turned up one Christmas with a tattoo on my head, and they were all, 'Oh no, he's gone forever, he's never coming back to normal society!' But then, all this happened, we started making a bit of cash, I bought a house - that kinda stuff really turns a parent's head," he grins. "So could you send me a copy of this when it runs? Because being in the Guardian is even more ammunition for me to use, to impress the parentals."